Due to its characteristics of being easily applicable to various products and electrical characteristics such as a high energy density, a secondary battery is not only commonly applied to a portable device, but universally applied to an electric vehicle (EV) or a hybrid electric vehicle (HEV) propelled by an electric driving source. This secondary battery is gaining attention for its primary advantages of remarkably reducing the use of fossil fuels and not generating by-products from the use of energy, making it a new eco-friendly and energy efficient source of energy.
Currently available secondary batteries include lithium ion batteries, lithium polymer batteries, Ni—Cd batteries, Ni-MH batteries and Ni—Zn batteries. An operating voltage of such a unit secondary battery cell is about 2.5V-4.2V. When a higher output voltage is required, a plurality of secondary battery cells may be connected in series to construct a battery pack. Also, based on a charge/discharge capacity required for a battery pack, a plurality of secondary battery cells may be connected in parallel to construct a battery pack. Thus, a number of secondary battery cells included in the battery pack may be variously set based on an output voltage or charge/discharge capacity required.
When constructing a battery pack by connecting a plurality of secondary battery cells are connected in series/in parallel, a general method involves constructing a battery module including at least two secondary battery cells, constructing a battery module assembly using a plurality of battery modules, and adding various components to the battery module assembly to construct a battery pack. With regard to the battery module, its example is disclosed in the Applicant's own Korean Patent Application Publication Nos. 10-2007-0064221 and 10-2010-0128927.